The present invention relates to a glow plug for preheating a subcombustion or combustion chamber of a diesel engine and, more particularly, to an improvement of a diesel engine glow plug of a bipolar two-line system having a ceramic heater which has high-speed and self temperature saturation properties and which allows "after glow" operation for a long period of time.
Conventional glow plugs having various types of structures have been proposed. Among these glow plugs, a plug having a ceramic heater has received a great deal of attention as a fast heating plug.
A glow plug of a ceramic heater type is described in Japanese Patent Laid-Open No. 60-14784. This glow plug has a structure wherein a heating element is exposed on the outer surface of a heater by using a resistive ceramic material having substantially the same thermal expansion coefficient as that of an insulating ceramic material constituting a heater insulating element, so that the heating element is integrally formed with the heater insulating element. With this structure, the distal end of the heater can be immediately heated to obtain a fast heating glow plug. At the same time, bonding between the heating element and the heater insulating element can be optimally and appropriately maintained to improve reliability for heat resistance or the like to some extent.
In a conventional glow plug of a ceramic heater type having the structure as described above, however, many problems are left unsolved from the structural and functional points of view when such a plug is used as a practical glow plug.
In a glow plug of this type, the distal end of the heater can be immediately heated to obtain a high-performance fast heating glow plug. At the same time, smooth and appropriate combustion inside the engine can be achieved due to recent improvements in the starting characteristics of the diesel engine and in durability for high-temperature operating conditions caused by turbo mechanization of diesel engines, as well as maintenance of an energizing state of the glow plug for a predetermined period of time after starting of the engine. This combustion condition allows reduction of an exhaust gas and noise. Market demand has arisen for such an after glow system, and maximum prolongation of the after glow time (e.g., 10 minutes) is required. In order to prolong the after glow time, energization power to the heating element must be self-controlled to greatly improve the heating characteristics, overheating of the heater portion must be prevented, and a self temperature saturation function is required to keep the saturation temperature below an appropriate temperature. At the time of after glow operation, a voltage applied to the glow plug is kept lower than that applied at the time of energization of the plug so as to assure durability of the heating wire. The conventional ceramic heater cannot satisfy the above needs. In a conventional heater structure, the heating portion of the distal end of the heater is generally made thin to obtain the fast heating properties. With this structure, however, the self temperature saturation properties cannot be obtained. In addition, the heat-resistive strength of the conventional structure is not satisfactory either. In consideration of these points, demand has arisen for providing countermeasures to obtain particularly self temperature saturation properties.